This invention relates to current limiting circuits and more particularly to limiting the current in a balanced two wire bidirectional transmission path that is operatively coupled to a pair of unbalanced unidirectional transmission paths.
In many instances it is desirable to limit the amount of current that can flow through a load. An undesirable high current condition can be caused by a short circuit or by the load having a variable impedance. For example, hybrid circuits have long been used as an interface between a pair of unbalanced unidirectional transmission paths and a balanced two wire bidirectional transmission path, or loop. The hybrid circuit is usually located at a central office while the bidirectional transmission loop is coupled to a subscriber telephone. The different distances of the various subscriber telephones from the central office cause the various subscriber loops to have different impedances. Accordingly, the subscriber loops for telephones located near the control office draw more current than for those telephones located further away which results in an inefficient use of available power. Also, due to falling objects such as trees, telephone poles and the like which result in accidental severing of the subscriber loop, the subscriber loop may become shorted thereby producing undesirably high currents. This is particularly undesirable when the hybrid circuit includes an integrated circuit which is inherently a low current device. Hybrid circuits which may include integrated circuit devices are shown and described in U.S. Pat. Nos. 4,004,109 and 3,849,609 the contents of which are incorporated herein by reference.
Accordingly, one object of this invention is to provide an improved current limiting circuit.
Another object of this invention is to provide an improved current limiting circuit which allows a large current to be regulated by regulating a small current and enable the use of integrated circuits in the low current path.
Still another object of this invention is to provide a current limiting circuit for a bidirectional current loop.
A still further object of this invention is to provide a current limiting circuit for a balanced bidirectional current loop that interfaces with a pair of unbalanced unidirectional transmission paths.